Tuesday 28 June 2011

Confirming the discovery of close approach asteroid 2011 MD

The Minor Planet Center (MPC) added two new LINEAR discoveries to the NEO Confirmation page (NEOCP) just after 01am UT on 23 June while I was taking images of another object discovered a day and a half earlier by the SPACEWATCH team (which would eventually be designated 2011 MF). With only 45 minutes left before twilight would get so bright that imaging would have to be abandoned, I decided to stop what I was doing and try for one of the LINEAR discoveries.

The two new objects were posted on the NEOCP with the temporary designations assigned to them by the LINEAR team, BZ52584 and BZ52587.

BZ52584 was in reasonably dark sky, about 4 degrees north of M13, the Great Globular star Cluster in Hercules while BZ52587 was much further east, about 3 degrees west of M31, the Andromeda galaxy and already in the glow of the approaching dawn. As BZ52584 was moving twice as fast as BZ52587 and better placed I decided it would be more useful and more likely to succeed to try and confirm it in the short time left before dawn. (BZ52587 would turn out to be a comet and be designated C/2011 M1 LINEAR a couple of days later).

However, the MPC prediction for BZ52584 was a bit odd - LINEAR had only observed it for 60 minutes some 18 hours earlier and normally the MPC would provide both a predicted position for the new object as well as an uncertainty map showing the likely area of sky the new object might be found in. However, this time only the predicted position was given, no uncertainty map. I took a set of images centered on the predicted position between 01:20 - 01:40 UT but when examined, there was no trace of the new object. With only about 20 minutes of usable sky left I started to hunt for BZ52584. With time only to take one or two more fields of view I chose to start with the field of view immediately to the east of the MPC predicted position.

As there was so little time left, the images were examined as soon as they were downloaded from the camera and after 7 minutes collecting 13 images it looked like there was a probable candidate, moving with the right motion, over 1/4 degree from the original prediction and just 17 pixels from the bottom of the images!

Two fields taken during the hunt for BZ52584. White denotes the first, centered on predicted position, yellow indicates the field where the object was found, very close to the bottom of the frame
One or two of the images had been spoiled by clouds that had started to thicken up and in a desperate attempt to positively confirm the new discovery before being clouded out I repositioned the telescope to centre the suspected object in the field of view and get some more images. The clouds continued to thicken and only 5 of 23 images taken after repositioning were at all usable, but fortunately they did show the new object in the expected place.

All of the good images were then measured and positions sent off to the MPC at 02:05 UT.

Animation of 2 sets of 5 x 20 second stacked exposures, showing motion of 2011 MD
01:41-01:45 UT 23 June 2011

The three positions I had just measured and the four provided by LINEAR were then put into FindOrb to work out an orbit and to provide an early view of where the new object was going to be in the next few days. It was immediately apparent that it was headed for a very close approach to Earth in 4 or 5 days time and so, to alert other observers and the MPC, I posted on the MPC's NEOCP blog at 02:16 UT:

"J95: BZ52584 probable v. close approach on June 27.2 UT"

"FindOrb gives ~23,000Km on June 27.2 UT (leaving out 1 of 704 positions). Worth
some more follow-up."

With further positions measured from other observatories in the following hours, the new object was announced by the MPC as 2011 MD later on June 23 and the close approach turned out to be somewhat closer than that first prediction, at 18,700 km from the Earth's centre on June 27.7 UT, or just under 1 Earth diameter from the Earth's surface.